1. Field of the Invention
This invention relates to a method of compressing image signals carrying gradation by prediction encoding processing.
2. Description of the Prior Art
In recent years, image signals carrying gradations are stored in the form of digital signals on a recording medium such as an optical disk, and read from the recording medium when necessary for reproducing a visible image on a cathode ray tube (CRT) or the like. Or, the image signals are transmitted in the form of digital signals by facsimile or the like, and a visible image is reproduced on the signal receiving side based on the received image signals.
In the case where image signals are stored or transmitted in the form of digital signals, it is desirable that the amount of the digital image signals be decreased in advance. This is because, in the case of image signals carrying gradations, the amount of the digital image signals is generally very large, and the signal storing cost, the signal transmission cost, and the like becomes markedly high when the digital image signals are directly stored or transmitted.
Under the above circumstances, in the case where the image signals are to be stored or transmitted, there has heretofore been used a method wherein compression processing based on prediction encoding is carried out to compress and decrease the amount of the image signals, and the compressed image signals are stored or transmitted. When the image which the image signals represent is to be reproduced, decoding processing is carried out on the compressed image signals to expand them, and a visible image is reproduced based on the expanded image signals.
Since the purpose of image signal compression processing based on prediction encoding is to decrease the amount of the image signals which are to be, for example, stored or transmitted, the image signal compressibility should naturally be as high as possible.
One approach to improving the compressibility in the case where image signal compression is carried out by prediction encoding is to decrease the bit resolution (density resolution) of the image signals, i.e. to carry out quantization processing for quantizing the image signals more coarsely, in addition to the prediction encoding processing.
However, when the bit resolution is decreased by quantizing the image signals coarsely, though the image signal compressibility can be improved, the image quality of the reproduced visible image deteriorates, and the reproduced visible image becomes visually unsuitable for viewing purposes. Also, in the case where the reproduced visible image is to be used for medical diagnosis, the diagnostic efficiency and accuracy become low.